Charge and spin photogalvanic effects in the p-wave magnet NiI2

TL;DR

This paper demonstrates how nonlinear optical transport in NiI2 reveals its unique p-wave magnetic states and spin textures, enabling direct optical probing and control of spin and charge currents.

Contribution

It establishes that nonlinear optical effects can directly probe and distinguish the spin spiral-induced inversion breaking and p-wave spin textures in NiI2.

Findings

Photogalvanic shift current exceeds that of conventional ferroelectrics.

Circularly polarized light induces helicity-selective spin transitions.

Pure spin currents can be generated and controlled without charge flow.

Abstract

NiI2 is an exotic van der Waals material in which a noncollinear spin spiral breaks spatial inversion symmetry without sizeable structural distortion, generating improper ferroelectric polarization, and stabilizing p-wave magnetic states with electron-volt-scale odd-parity spin splitting. Using first-principles calculations, here we establish that nonlinear optical transport can directly probe and separate these effects. Magnetically-induced inversion breaking associated with the spin spiral produces a photogalvanic shift current under linearly polarized light, with conductivities exceeding those of conventional ferroelectrics. In contrast, a large photogalvanic injection current under circularly polarized light originates from helicity-selective transitions between spin-split states at opposite crystal momenta, directly exposing the nonrelativistic p-wave spin texture. We further predict pure spin photocurrents whose flow direction exchanges with that of the charge current under linear and circular excitation. The ability to generate and control pure spin currents without accompanying charge currents makes NiI2 a promising material platform for all-optical spin injection in van der Waals heterostructures.